The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Oxides of nitrogen, NOx, are known by-products of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures and engine air/fuel ratio. Additionally, aftertreatment devices include materials, such as catalyst beds, prone to damage or degradation as a result of use over time and exposure to high temperatures.
Engine control methods may utilize diverse operating strategies to optimize combustion. Some operating strategies optimizing combustion in terms of fuel efficiency include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder and increase engine efficiency, for example, by operating in an unthrottled condition, reducing air intake pumping losses. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies since aftertreatment devices frequently require an elevated operating temperature driven by the exhaust gas flow temperature to operate adequately to treat NOx emissions.
Aftertreatment devices are known, for instance, utilizing chemical reactions to treat exhaust gas flow. One exemplary device includes a selective catalytic reduction device (SCR). Known uses of an SCR device utilize ammonia derived from urea injection to treat NOx. Ammonia stored on a catalyst bed within the SCR reacts with NOx, preferably in a desired proportion of NO and NO2, and produces favorable reactions to treat the NOx. One exemplary embodiment includes a preferred one to one, NO2 to NO molar proportion, and is known as a fast SCR reaction. It is known to operate a NOx treatment catalyst such as a diesel oxidation catalyst (DOC) upstream of the SCR in diesel applications to convert NO into NO2 for preferable treatment in the SCR. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust gas flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
Other aftertreatment devices are additionally known for treating the exhaust gas flow. NOx treatment catalysts, such as three way catalysts (TWC) are utilized particularly in gasoline applications. Lean NOx traps (NOx trap) utilize catalysts capable of storing some amount of NOx, and engine control technologies have been developed to combine these NOx traps or NOx absorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a lean NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions with conventional three-way catalysis to nitrogen and water. However, storing NOx during lower temperature engine operating conditions with conventional three-way catalysts limits NOx storage to exhaust gas feedstream NO2 with the NOx trap, when the temperature of the three-way catalyst is too low to convert exhaust gas feedstream NO to NO2. Diesel particulate filters (DPF) trap soot and particulate matter in diesel applications, and the trapped material is periodically purged in high temperature regeneration events. A high exhaust NO2/NO fraction assists in this purging.
It is also known in the art that engine modeling of various types is helpful in understanding and predicting behavior in engines. These models incorporate various levels of complexity in the description of the physical and chemical processes that occur during engine operation and during the operation of various exhaust emissions treatment devices. Models that incorporate a relatively simple description of the physical processes and a more detailed description of the chemical processes occurring during combustion can be very useful in describing and obtaining reasonable predictions of engine phenomena that are highly dependent on combustion chemistry, such as exhaust gas constituent formation and destruction in the engine and exhaust, autoignition, and conversion of NO to NO2 in an engine, while minimizing the cost and complexity involved in using the models.